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Night Blindness, Congenital Stationary, CSNB2B
Night blindness is a feature of many pigmentary and other retinal disorders, most of which are progressive. However, there is also a group of genetically heterogeneous disorders, with generally stable scotopic defects and without RPE changes, known as congenital stationary night blindness (CSNB). At least 9 mutant genes are responsible with phenotypes so similar that genotyping is usually necessary to distinguish them. All are caused by defective photoreceptor-to-bipolar cell signaling with common ERG findings of reduced or absent b-waves and generally normal a-waves. The photopic ERG is usually abnormal to some degree as well and visual acuity may be subnormal. In the pregenomic era, subtleties of ERG responses were frequently used in an attempt to distinguish different forms of CSNB. Genotyping now enables classification with unprecedented precision.
In this disorder (CSNB2B) the b-wave responses are deficient (little or no scotopic response) and a-waves seem to be normal. However, many if not most patients do not complain of night blindness. Nystagmus, strabismus, and restriction of visual fields may be present. Visual acuity is mildly to severely reduced.
No systemic disease is associated with congenital stationary night blindness.
CSNB2B, or type 2B, is one of four congenital nightblindness disorders with autosomal recessive inheritance. It results from mutations in the CAPB4 (11q13.1) gene important in receptor to bipolar cell signaling.
No treatment beyond correction of the refractive error is available but tinted lenses are sometimes used to enhance vision.